1. Field of the Invention
The present invention relates to a device for detecting an electrostatic capacitance formed by an object to be detected and a detecting electrode and, more specifically, to an electrostatic capacitance detecting device usable to electrically obtain a fingerprint pattern by detecting difference in electrostatic capacitance due to projections and depressions of a fingerprint independent of initially charged condition of a finger that is an object to be detected.
2. Description of the Related Art
With development of the information society, individual identification by fingerprint collation is an easy and reliable security check means for prevention of unauthorized log-in into networks, identification in electronic commerce, identification in various administrative systems, prevention of use of credit cards and the like by unauthorized people.
FIG. 10 is a basic configuration diagram of a fingerprint input device which has been conventionally widely used.
In this device, a light emitting diode (LED) 71 emits light to a prism 72 with which a finger 70 is in contact. The light is reflected on a reflection surface, with which the finger is in contact, of the prism 72 to deflect by 90xc2x0 toward a lens 73. Then, the lens 73 gathers light signals containing the information of projections and depressions of a fingerprint of the finger 70 to form a picture image on a light-receptive surface of a CCD (Charge Coupled Device) 74 which is an image pickup device. The CCD 74 converts distribution of light strength of the formed image into electrical signals. This makes it possible to obtain electrical fingerprint information.
The conventional fingerprint input device as described above, however, is large in volume and weight, and, further, is costly.
With development of the information society, terminal devices, like cellular phones which can get access while moving have also appeared as devices located at the ends of the networks, and thus problems concerning security checks for the networks have been becoming more serious.
However, it is difficult to mount the above-described conventional fingerprint input device to a portable terminal device for the security check.
More specifically, the conventional fingerprint input device shown in FIG. 10 needs a comparatively large prism and lens to optically catch an image of a fingerprint, and they and a light source need to be arranged three-dimensionally, naturally resulting in a large and heavy fingerprint input device.
Further, the conventional fingerprint input device is costly because it requires many assembly tasks, such as precise optical alignment, which need advanced techniques, and it has a high parts count.
Furthermore, driving the CCD, which is an image pickup device, generally requires three power sources and power consumption of several hundred milliwatts. Moreover, the conventional fingerprint input device requires electric power for causing the LED to emit light, and thus it is difficult to use the above device in a portable device driven by a battery.
Thus, for example, a flat-shaped fingerprint input device by an electrostatic capacitance detection method is supposed as viewed in Japan Patent Laid-open No. 8-305832. This device is constituted by arranging many detecting electrodes and switch elements on a substrate in two dimensions and connecting them to an electrostatic capacitance detecting circuit and a driving circuit with Y wirings and X wirings.
When a finger is put on the device to detect an electrostatic capacitance between the device and the finger, one Y wiring is first turned OFF to pre-charge a parasitic capacitance of the X wiring to a potential. Next, the Y wiring is turned ON to distribute the charges between the electrostatic capacitance formed between the finger and the detecting electrode, and the aforesaid parasitic capacitance, thereby detecting the electrostatic capacitance between the finger and the device by change in potential of the X wiring at this time. Then, detection of electrostatic detecting capacitances between many detecting electrodes and the finger enables input of the information of the fingerprint.
In the conventional device as described above, however, the change in potential of the X wiring at the time of charge distribution by the electrostatic capacitance between the finger and the detecting electrode, and the parasitic capacitance depends on initially charged condition of the finger. The finger varies in charged condition, making it difficult to detect the accurate electrostatic capacitance.
Further, charges need to be injected into the X wiring and the parasitic capacitance for readout so as to pre-charge for every detection of the electrostatic capacitance, and thus there are problems that the detection speed is low and that the movement the finger during the detection makes it impossible to input an accurate electrostatic capacitance distribution in accordance with the fingerprint.
Therefore, a suggestion for solving such problems in the finger print input device by the electrostatic capacitance detection method is made in Japan Patent Laid-open No. 11-19070. More specifically, electrodes formed in a mesh or in a comb are provided around many detecting electrodes, and a radio frequency is applied to the electrodes by a radio frequency generator to emit a radio frequency toward a finger. This enables detection of the accurate electrostatic capacitance without being affected by the charged condition of the finger.
However, the provision of the electrodes formed in a mesh or in a comb and the necessity of the high frequency generator results in high cost and in disadvantage for making the device smaller and thinner. Further, there is a problem that it is difficult to speed up readout and detection of the electrostatic capacitance as in the above-described conventional example.
The present invention is carried out to solve the above problems, and its object is to provide an electrostatic capacitance detecting device which is small-sized, thin, lightweight with low power consumption, and additionally is capable of being fabricated at a low cost, and, more specifically, to provide an electrostatic capacitance detecting device capable of electrically obtaining a fingerprint pattern at a high speed without being affected by an initial potential of an object to be detected, and mountable on a portable device.
To achieve the above-described object, the present invention provides an electrostatic capacitance detecting device configured as follows.
On a semiconductor substrate, a detecting electrode constituting an electrostatic capacitance element by being brought into contact with or put close to an object to be detected through a protection film; a bias switch element for injecting bias charges into the detecting electrode; a charge transfer switch element for transferring signal charges accumulated in the detecting electrode in response to a timing signal for signal detection; a capacitance element for converting the signal charges transferred thereto via the charge transfer switch element into a voltage signal; a source follower amplifier element for receiving and amplifying a holding voltage of the capacitance element, a readout selection switch element provided on a source side of the source follower amplifier element; and a reset switch element providing the capacitance element with a reset potential, are provided.
Further, it is possible to constitute an electrostatic capacitance detecting device comprising a detecting area constituted by arranging two-dimensionally a plurality of the electrostatic capacitance detecting devices each of which is formed by the above-described configuration as an individual detecting cell, readout row selection means for selecting a readout row, and readout column selection means for selecting a readout column in the detecting area.
In this case, it is preferable that the readout row selection means is a first shift register for controlling the readout selection switch element of each detecting cell for each readout row, and that the readout column selection means is composed of vertical signal lines for sending-out output signals from the readout selection switch elements of the detecting cells together for each readout column, a single detection signal output line, column selection switch elements provided between the vertical signal lines and the single detection signal output line respectively, and a second shift register for controlling the column selection switch elements.
It is preferable that each of the bias switch element, the charge transfer switch element, the reset switch element, the source follower amplifier element, and the readout selection switch element is composed of a MOS transistor.
Furthermore, the reset switch elements, the reset power supply line, and the reset gate control line in the above-described electrostatic capacitance detecting device can be omitted. Alternatively, the bias switch elements, the bias power supply line, and the bias gate control line may be omitted.
In the electrostatic capacitance detecting device according to the invention, the object to be detected and the detecting electrode constitute the electrostatic capacitance element in accordance with projections and depressions of the object to be detected, charges are injected into the electrostatic capacitance element to thereby detect its signal by a detection circuit provided in each detecting cell, and the signal is amplified by the source follower amplifier element to be outputted.
In this event, the detecting electrode forming the electrostatic capacitance element with the object to be detected holds a prescribed potential before the detection of the signal, enabling the detection of accurate capacitance without being affected by the initially charged condition of the object to be detected.
Even in the configuration in which a plurality of the detecting cells are arranged two-dimensionally to constitute the detecting area, charges are injected into each electrostatic capacitance element in accordance with projections and depressions of the object to be detected and the signal is detected in each unit cell, thereby making it possible to operate the electrostatic capacitance detecting device at a speed higher than a conventional one which detects a signal by injecting charges into wiring for readout and its parasitic capacitance. Further, the power consumption is also low.
The above and other objects, features and advantages of the invention will be apparent from the following detailed description which is to be read in conjunction with the accompanying drawings.